Yesterday, naturalist Sir David Attenborough participated in a reddit AMA, answering questions from the reddit community. One thing that he revealed during the Q&A was the most upsetting thing he has witnessed in his career: chimpanzees hunting monkeys.

Hello, Sir David!
I remember feeling absolutely heartbroken watching the elephant calf die in Africa. What has been your most distressing/upsetting moment in your career?

To which Attenborough replied:

Seeing chimpanzees kill monkeys, they do this to eat them. They chase them, set an ambush, catch them, and tear them apart.

Up top is a video chimpanzees in Uganda hunting monkeys, from BBC's Life of Mammals (via nrrrdgrrl), and you can hear the alarm in Attenborough's voice as he narrates the sequence. Head over to reddit to read the rest of the AMA, including a portrait of Attenborough, courtesy of user Shitty_Watercolour.